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Mr Walker said finding the invaders was challenging because Norway rats hid during the day in burrows.

"This is where the rodent detection dogs and their handlers are key."

DoC was still trying to work out how a rat got on to Okahu Island. Possibilities include swimming or hitching a ride on a boat.

Two of the rats were young females which did not appear to have bred. DNA samples had been sent away for analysis in a bid to find out where the rodents came from.

Even one rat could do a lot of damage by eating birds, eggs, reptiles and insects, Mr Walker said. He urged island visitors to check their boats and gear for rats, mice, ants and skinks to avoid accidentally re-introducing pests.

Project Island Song aims to restore native wildlife on the Ipipiri islands. It is driven by community group Guardians of the Bay of Islands in partnership with DoC, Rawhiti hapu and private landowners.

Species released so far are tieke, popokotea, pateke (brown teal) and toutouwai (North Island robins).

Rat numbers may have been boosted this year by a wet summer which has ensured plenty of food for native birds but also for introduced pests.